The invention generally relates to implantable cardiac stimulation devices such as implantable cardioverter defibrillators (ICDS) and in particular to power transformers for use within implantable cardiac stimulation devices.
An ICD is a type of implantable cardiac stimulation device which recognizes atrial fibrillation or ventricular fibrillation in the heart of a patient and delivers electrical shocks to terminate the fibrillation. Typically, defibrillation shocks are delivered by a pulse delivery circuit connected to a pair of capacitors charged at a combined voltage of about 1500 volts. In response to the detection of fibrillation, the pulse delivery circuit discharges the capacitors to provide a defibrillation pulse having a desired shape and voltage for applying to the heart. The capacitors should be fully charged before the defibrillation pulse is generated. The power supply of the ICD is typically a battery generating a voltage of only about 3.3 volts. Hence, one or more flyback transformer are provided between the power supply and the pair of defibrillation capacitors to incrementally charge the defibrillation capacitors prior to delivery of a defibrillation pulse. In many ICDs, the pulse delivery circuit includes a set of four insulated gate bipolar transistors (IGBTs) formed in an H-bridge configuration. The IGBTs are switched on and off so as to apply charge stored in the capacitors in biphasic pulse waveform to the heart. To switch an IGBT on, a voltage of about 15 volts usually needs to be applied to a gate of the IGBT. Hence, a pair of non-flyback transformers are provided for converting the voltage provided by the power supply to the voltage required to switch on the IGBTs of the pulse delivery circuit.
In any implantable cardiac stimulation device, particularly, ICDs, it is critical that the size of the device be minimized and that reliability be maximized. Size must be minimized to make the device as small and light as possible to reduce discomfort to the patient after the device has been implanted. Reliability must be maximized to ensure that the patient receives the appropriate therapy at all times and that the device need not be explanted from the patient to replace malfunctioning components. As can be appreciated, if an ICD fails to deliver adequate defibrillation therapy, the patient may not survive an episode of ventricular fibrillation. Unfortunately, conventional techniques for implementing transformers with ICDs neither minimize device size or maximize device reliability.
In particular, the flyback and non-flyback transformers for use in ICDs are typically configured using transformer coils and core members which are physically separate from one another and from other electrical components of the device, such as from printed circuit boards (PCBs) containing capacitors, switching transistors and the like. The transformer coils and core members consume considerable volume individually and, since they are separate from other components, additional space is needed to accommodate the necessary electrical interconnections. Hence, size is not minimized. Moreover, with transformer coils and core members installed separately from other components, reliability is not optimal as electrical interconnection problems could arise during fabrication or during operation of the device. In particular, given the high voltage generated by the flyback transformers, heat generated by electrical resistance could possibly damage electrical interconnections between the flyback transformer coil and other components of the device causing the device to fail.
Thus, it would be highly desirable to provide improved transformer assemblies for use within ICDs or other implantable cardiac stimulation devices which addresses the aforementioned concerns and it is to that end that aspects of the invention are primarily directed.
In accordance with the invention, an improved transformer assembly is provided for use in an ICD or other implantable cardiac stimulation device. The transformer assembly is coupled between a power supply and a pulse delivery circuit for transforming the voltage of the power supply to voltages required by the pulse delivery circuit. The power transformer assembly is formed using a PCB having transformer coil windings or turns embedded therein with one or more transformer cores mounted adjacent to the windings of the PCB.
In an exemplary embodiment, the transformer cores are generally planar ferrite cores with a set of feet for mounting into a set of apertures formed in a multi-layer PCB. The coils of the transformers are embedded within the layers of the PCB with one or more coil turns or loops per layer. Other circuit components such as capacitors, transistors and the like may also be mounted to or embedded in the PCB. By embedding the coil windings of the transformers within the PCB, the transformers may be more easily integrated with other circuit components so that the overall size of the stimulation device may be reduced. Moreover, the use of a PCB helps avoid reliability problems that might otherwise occur in the fabrication, assembly and operation of the device. For example, a switching transistor for controlling the operation of a transformer can be mounted to the same PCB as the transformer coil thereby eliminating the need for potentially bulky device interconnection lines between the switching transistor and the transformer coil thereby reducing size and improving reliability.
In a specific exemplary embodiment, a primary and two secondary transformers are provided for use with a twelve-layer PCB. The primary transformer is used as a flyback transformer for charging a pair of defibrillation capacitors. The two secondary transformers are used to provide voltage for selectively switching on a set of four IGBTs formed in an H-bridge configuration so as to apply charge stored in the capacitors in biphasic pulse waveform to the heart. The coil of the flyback transformer is embedded within the four middle layers of the twelve-layer PCB within one turn per layer. The coils of the two secondary transformers are embedded within the top four and bottom four layers, respectively, of the PCB, with three turns per layer.
In another specific exemplary embodiment, wherein a primary and two secondary transformers are also provided, only the coil of the primary transformer is embedded within the PCB. The coils of the two secondary transformers are affixed to the surface of the PCB. As with the preceding embodiment, the primary transformer is used as a flyback transformer and the two secondary transformers are used to provide voltage to H-bridge switching IGBTs for applying biphasic pulse waveforms to the heart.
Numerous other embodiments are consistent with the invention as well. Other embodiments, advantage and features of the invention are discussed below.